Exploring the interactions of apatitic tricalcium phosphate with copper ions: Time-resolved structural transformations and electrical property insights

This study investigates the dynamic transformation of apatitic tricalcium phosphate (Ca9(HPO4)(PO4)5(OH)) when exposed to copper ions (Cu2+) over varying contact time. Characterization techniques, including XRD and ICP-OES confirm the initial formation of copper hydroxyphosphate (Cu2(PO4)(OH)) after 8 h, coexisting with Ca9(HPO4)(PO4)5(OH). Surprisingly, after 120 h, the reaction progressed yielding exclusive Cu2(PO4)(OH) and complete conversion of the initial apatitic tricalcium phosphate. Impedance spectroscopy revealed a time-dependent increase in resistance, suggesting changes in both bulk and interfacial electrical properties. The dominant relaxation observed at higher frequencies suggests grain-related processes, while a secondary relaxation at medium frequencies indicates the influence of grain boundary-related processes. Likewise, at lower frequencies, the observed relaxation suggests the influence of diffusion processes related to the introduction of Cu2+. The evolution of relaxation times (τ) further supported the changing electrochemical behavior over time. AC conductivity measurements demonstrated a frequency-independent plateau at high frequencies, with decreasing conductivity over time, possibly indicative of evolving microstructural features or altered charge transport pathways. This comprehensive investigation provides valuable insights into the complex interplay of electrochemical and structural changes during the intriguing transformation of Ca9(HPO4)(PO4)5(OH) into Cu2(PO4)(OH), enhancing our understanding of similar systems and offering potential applications in copper ion removal, as well as exploring Cu2(PO4)(OH) applications.